1

Mass is one of the most important quantities of the physico-mechanical system. The concept of mass constitutes a universal characteristic of bodies. Newton described mass as a physical quantity that could be determined in relationship with other physical quantities. Experimentally, it was found that if the same force was applied to different bodies, they showed different acceleration directly proportional to their mass. As a result, we see traceablity related to mass in the fields of force and pressure as well.

Mass is a key measurement in the world of trade, so much so that there is an organization known as OIML, the International Organisation of Legal Metrology, which is focusesd on lLegal mMetrology, or that metrology which effects the daily trade of goods. Obviously this also applies to the trade between Countriescountries, and as a result, the OIML hasve been party to defining certain regulations and guidelines related to mass standards and measurements. The most important for the mass industry is the R 111, which is available from the OIML.

Mass is defined as the mass of the iInternational pPrototype of the kilogram kept under the custody of the International Bureau of Weights & Measures (BIPM) at Paris in France. It is a right circular cylinder of height 39 mm and equal diameter. It is made of single-phase alloy of platinum-iridium with 10% iridium by weight. Currently the prototype kilogram is the last remaining artifact that forms part of the SI units. Work is currently underway to redefine the kilogram in terms of an invariant of nature. Till then, many cCountries have copies of the prototype that are regularly inter-compared with the standard artifact at the BIPM. The SI unit of mass is known as the kilogram (kg).

This document covers the establishment of a mass metrology laboratory for calibration of weights generally required by Industrial level laboratories and the industry.

The OIML has defined certain classes of weights and given a brief description of each. In summary, they are the following
Class E1 - Inter-comparison between international mass standards & calibration of class E2 weights.
Class E2 - Typical laboratory standards to be used to calibrate class F1 weights
Class F1 - Used to calibrate class F2 weights & class 1 accuracy instruments
Class F2 - Used to calibrate class M1 & M2 weights & class 2 accuracy instruments
Class M1 - General work with class 2 accuracy weighing instruments
Class M2 - General work with class 3 accuracy weighing instruments
Class M3 - General work with class 3 & 4 accuracy weighing instruments

Equipment Required

Mass comparators and balances are used to inter-compare the various levels of mass pieces and so the traceablity chain is maintained. Lower accuracy devices are typically referred to as weighing instruments, and these range from tiny scales right through to heavy vehicle weigh bridges. The laboratory needs to select the correct instruments to calibrate the requirements from industry and then the class of mass pieces chosen to be the laboratory standards. They need to be at least one class above the industry requirements.

Normally all weights are made in compliance with the OIML International Recommendation R111, which prescribes constructional requirements and specified materials, tolerances, surface conditions, density and markings. The highest precision weights, Classes E1 and E2, are solid stainless steel, with a prescribed density. These have no weight markings as the marking would itself attract dirt. Lower grade weights are usually adjustable and may be of materials other than stainless steel, including chrome-plated brass, brass or painted cast iron. Except for the lowest grade of weights, stainless steel is the preferred material because of its several advantages as given below:-
(i) Stability
(ii) Ability to be polished during manufacture
(iii) Density
Smaller weights may be of stainless steel, germanium, nickel, silver or aluminium. Because of its density and softness, aluminium is only used for weights up to 10 mg. In an ideal case a calibration laboratory should have a minimum of two sets of standards, to ensure that there is no dependence on a single standard weight whose value may change between calibrations without the knowledge of the laboratory.
For higher accuracy work, a simple one-to-one calibration is sufficient, with calibration in groups using a restraint or check standard (usually 1, 10 or 100 level). When a large number of low accuracy weights are calibrated, it may be necessary to have other low grade weights to avoid wear on the laboratory's main standards

Tolerance limits

4.c. Maximum possible errors on verification for conventional measures
Maximum permissible errors for weights (± dm in mg)

The nominal weight values in the table specify the smallest and largest weight permitted in any class of R 111 and the maximum permissible errors and denominations shall not be extrapolated to higher or lower values. For example, the smallest nominal value for a weight in class M2 is 100 mg while the largest is 5 000 kg. A 50 mg weight would not be accepted as an R 111 class M2 weight and instead should meet class M1 maximum permissible errors and other requirements (e.g. shape or markings) for that class of weight. Otherwise the weight cannot be described as complying with R 111.